After , a and b are miai to kill (?); if
at one,
at the other. [1]
If and
go at two of the circled points, then
and
at the squared points.
at the remaining circled point captures, but...
here kills the black group.
Notice that in the first diagram at a or b allows
at
there, which costs white an additional move in connecting and black can either retake the vital point (making eyes) or push outwards through the remaining outside liberty. [2]
[1]
unkx80: After and
, Black will tenuki. Isn't this a seki?
JoelR: Indeed. White can't threaten to kill without making a capturable live shape. On the other hand,
This takes away Black's option, and now even if Black makes no more plays, White can play at the squares for atari, with a bulky five dead shape inside.
[2]
JoelR: The throw-in doesn't help Black---after closing off the outside, White can play at the square to make atari and a dead shape. If Black tries to push out with 'a', then 'b' keeps him in.
unkx80: Yes, this is what I am looking for. =)